For good service, the surface of a floor should be its hardest part. However, the nature of plastic concrete and the finishing process for floors can tend to make the surface the weakest section of a slab. Fortunately, there are several straightforward technique for combating this undesirable tendency. The quality of the surface depends on the hardness or resiliency of the materials at the. Surface, how densely they are compacted by finishing and how firmly they are bound. The materials at the surface of a concrete slab tend to have slightly different proportions form the materials in the body of the floor. This comes from two causes. One is the tendency of heavier materials like cement and normal weight aggregate to sink and of lighter materials like water and lightweight coarse aggregate to rise to the surface. The other is the large amount of work that is applied to the surface during placing and finishing. The water performs two major functions. First it helps make the mix plastic and workable. Second, it reacts chemically with portland cement to form new compounds which become the solid binders that hold the mass together. These two functions of water in a mix can produce a conflict: while water is necessary to the chemical change which hardens concrete, too much water in the mix produces a leaner cement binder and weakens the slab. How much is too much? It seems too often to be that amount which the contractor persuades the ready mix producer to add to the mix in order to increase workability. Workability can be improved without adding water by improvement is mix proportions or through the use of a water reducing admixture. The proper use of vibrators also can help get a stiff mix into place without sacrificing strength through the addition of water.